Recently, LCD devices capable of displaying three-dimensional images have been developed actively. There are many techniques for displaying three-dimensional images. Among them, much attention has been paid to a technique for displaying three-dimensional images on a single liquid crystal display panel using time-division drive.
FIG. 5 is a view schematically showing a procedure of a conventional LCD device displaying a three-dimensional image by time-division drive. As shown in the drawing, when displaying a three-dimensional image by time-division drive, one frame period is divided into a plurality of sub-frame periods. In the example shown in FIG. 5, one frame period is divided into four sub-frame periods. In a case of driving a liquid crystal display panel at frame frequency of 60 Hz, one frame period is approximately 16.7 ms, so that one sub-frame period is approximately 4.2 ms. Accordingly, a right-eye image or a left-eye image is displayed per one sub-frame period (4.2 ms). In both cases of displaying a right-eye image and a left-eye image, the screen of the liquid crystal display panel is scanned from the upper portion thereof to the lower portion thereof.
In the example shown in FIG. 5, both in two sub-frame periods, a right-eye image is displayed, and both in subsequent two sub-frame periods, a left-eye image is displayed. At that time, while an image signal for the right-eye image is written in pixels, a backlight is turned off. Furthermore, at that time, the right-eye shutter glass is made off. Consequently, the right eye of the user does not see the right-eye image while it is written.
In the two successive sub-frame periods, at timing when an image signal for the right-eye image is completely written in pixels during the former sub-frame period, the right-eye shutter glass is put in an on-state. At that time, the backlight is still kept in the off-state. At timing when an image signal for the right-eye image is completely written in pixels during the latter sub-frame period, the backlight is put in an on-state. Consequently, a left-eye image is displayed on the whole screen at once, and the left-eye image is seen by the left eye of the user. The period in which the backlight is turned on is shorter than the sub-frame period. That is, flash light is outputted from the backlight.
After the right-eye image is completely displayed, a left-eye image is displayed in remaining two sub-frame periods in the same frame period. Specifically, in the two successive sub-frame periods, at timing when an image signal for the left-eye image is completely written in pixels during the former sub-frame period, the left-eye shutter glass is put in an on-state. At that time, the backlight is still kept in the off-state. At timing when an image signal for the left-eye image is completely written in pixels during the latter sub-frame period, the backlight is turned on. Consequently, a left-eye image is displayed on the whole screen at once, and the left-eye image is seen by the left eye of the user. The period in which the backlight is turned on is set to be shorter than the sub-frame period. That is, flash light is outputted from the backlight.
In theory, the above method enables a single liquid crystal display panel to appropriately display a three-dimensional image. However, in reality, there is a problem of crosstalk between images due to insufficient response speed of liquid crystal.
Prior to explaining this problem, initially, an explanation is made as to response speed of liquid crystal. FIG. 6 is a view showing falling response of liquid crystal. A graph 61 shown in the drawing indicates a relation between relative luminance of light transmitted by liquid crystal and a time required for a change in the relative luminance. According to the graph 61, it takes approximately 4 ms for the relative luminance to change from a peak value to 10% of the peak value (luminance difference indicated by the graph 61 of FIG. 6). Normally, falling response of this level allows two-dimensional image display without drop in image quality.
On the other hand, in a case of displaying a three-dimensional image by time-division drive, the falling response corresponding to luminance difference indicated by the graph 61 of FIG. 6 is not sufficient. Specifically, there is required such a falling response that luminance drops to 0.1% of its peak value as indicated by a graph 62 of FIG. 6. As indicated by a graph 60 of FIG. 6, it takes approximately 10 ms for luminance to drop to 0.1% of its peak value (luminance difference indicated by graph 62 of FIG. 6). The falling response of this level does not provide sufficient response speed of liquid crystal, so that there arises a problem that occurrence of crosstalk in displaying three-dimensional images drops image quality.
This problem is explained in more detail with reference to FIG. 5. In time-division drive shown in FIG. 5, there is a sufficient time from when scanning of the upper portion of a screen is started to when a backlight is turned on. This indicates that there is a sufficient response time for liquid crystal. Accordingly, at timing when the backlight is turned on, a response of liquid crystal to which a voltage was applied has been completed, and so orientation of the liquid crystal has changed in such a degree that enables light with targeted luminance to be transmitted.
On the other hand, scanning of the lower portion of the screen is started later than scanning of the upper portion, and so a time from when scanning of the lower portion is started to when the backlight is turned on is shorter than a time when scanning of the upper portion is started to when the backlight is turned on. Consequently, the liquid crystal does not have a sufficient response time. As a result, at the lower portion of the screen, the liquid crystal to which a voltage was applied has not responded sufficiently or has not responded at all. Therefore, orientation of the liquid crystal does not correspond to the level of an image signal written in the present sub-frame period, but corresponds to the level of an image signal written in the previous sub-frame period.
Consequently, there arises a problem that at timing when the backlight is turned on, the upper portion of the screen correctly displays a targeted image, whereas the lower portion of the screen cannot display the targeted image and instead displays a previous image. When displaying a three-dimensional image by time-division drive, a right-eye image and a left-eye image are displayed alternately. Consequently, the above problem results in displaying an image in which the right-eye image and the left-eye image are mixed partially. In order that a viewer views an exact three-dimensional image, it is essential to alternately display a right-eye image and a left-eye image in such a manner that the right-eye image and the left-eye image are completely separated. Therefore, displaying an image in which a right-eye image and a left-eye image are mixed would fatally drop image quality of a three-dimensional image.
On the other hand, conventionally, attention has been paid to a relation between the temperature of liquid crystal and response speed, and there have been developed LCD devices using this relation. For example, Patent Literature 1 discloses an LCD device including two liquid crystal display panels at front and rear sides superimposed with each other with a predetermined distance therebetween so that an image displayed on the liquid crystal display panel at the front side and an image displayed on the liquid crystal display panel at the rear side are superimposed with each other to form a stereoscopic image, wherein response time characteristics of the liquid crystal display panels at the front and rear sides, respectively, are different from each other at the same ambient temperature.
With this LCD device, a configuration of a plurality of liquid crystal display panels at front and rear sides superimposed with each other to form a stereoscopic image does not cause blurring of an image due to the difference in temperature between the liquid crystal display panels at the front and rear sides.